Question

I have a game which displays an array of colored blocks. The user can move these blocks around the screen. When a touch event occurs, I take note of the cell that has been touched (cell[i][j].isMoving = true). If the user moves the block around I draw the rectangle relative to an offset value. When a touch up event is detected, I check whether or not the user has dragged the block far enough to signify a moving of a block.

My basic draw loop is as follows:

for(int i = 0; i < xCells, i++){
   for(int j = 0; j < yCells; j++){
       if(cell[i][j].isMoving)
             canvas.drawRect(...) // draw with offsets
       else
             canvas.drawRect(..)
      }
 }

The problem I am having is when a user releases the block it occasionally flickers briefly.

When a touch up event occurs, the offset must be set to 0, the coordinates of the block changed (if requirements are met) and isMoving has to be set to false.
As I have a thread constantly running that calls the draw code above, it appears that the UI thread is altering the array of blocks, meaning it is in an inconsistent state when the draw method occurs.

Does anyone have suggestions on how to fix this? Could I use a handler? I’ve tried synchronizing the onTouchEvent method and the onDraw method, but this seems to occasionally block user input

thanks

Answer 1

The safe solution is to only allow one thread to create objects, add and remove them from a List or array after the game has started.

I had problems with random AIOOBEs (Array Index Out Of Bounds Exception) errors and no synchornize could solve it properly plus it was slowing down the response of the user.

My solution, which is now stable and fast (never had an AIOOBEs since) is to make UI thread inform the game thread to create or manipulate an object by setting a boolean flag and coordinates of the touch into simple variables.

Since the game thread loops about 60 times per second this proved to be sufficent to pick up the message from the UI thread and do something about it.

In your case onTouch event should never alter cell[i][j] directly but only set some integer to a particluar value. Then your thread would at some point evaluate this variables and set cell[i][j] and then draw, so onTouch can never unpredictably interfear with your thread.

This is a very simple solution and it works great…